In at least some known systems, when a wafer, for example a silicon wafer, is placed in a Fizeau interferometer for analysis, the placement of the wafer causes the wafer to vibrate. It is possible to perform Fourier based phase shift interferometry to suppress measurement errors caused by vibrations of the wafer, but the large amount of numerical processing required for such Fourier based phase shift interferometry makes such an approach impractical. In contrast, some known systems perform phase shift interferometry using a limited set of wavelengths distributed over one interference period. By reducing the number of wavelengths in the set, the accuracy of the phase shift interferometry is similarly reduced, while processing speed is increased. In other systems for suppressing errors from residual vibrations of the wafer, a wavelength is continuously scanned. During the wavelength scan, a camera captures a series of images, producing reference wavelength images for an algorithm designed for a limited set of wavelengths. However, exposure is timed in such a way that each reference image is an integral (i.e., in-camera integration) of a periodically-changing interference signal. In addition, the image capture process is repeated a predefined number of times (i.e., n-times). Subsequently, the sets of images are averaged before applying the algorithm designed for the limited wavelength set. In-camera integration and n-times averaging suppress image sensor noise and residual vibration of the wafer. However, this approach still suffers from an inherent lack of numerical accuracy, due to the reliance on an algorithm designed for use with a limited wavelength set. Additionally, the approach requires waiting an indefinite amount of time for the vibrations of the wafer to stop before performing image capture. Known systems have no ability to detect the quality of data or level of vibrations. Another drawback of such systems is the need for absolute wavelength calibration, to enable the n-times averaging, as even a relatively slight wavelength miscalibration may result in relatively significant numerical errors.
This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.